There are, however, certain crystals that with a moderate thickness give an emergent stream of light that is more or less completely polarized.
In general a stream of plane-polarized light undergoes no change in traversing a plate of an uniaxal crystal in the direction of its axis, and when the emergent stream is analysed, the light, if originally white, is found to be colourless and to be extinguished when the polarizer and analyser are crossed.
On the other hand, if the emergent streams overlap and the common part be examined, it is found to have all the properties of common light.
Telescope has been adjusted to focus upon a star, and to divide the diameter of the object-, glass by the diameter of the emergent pencil.
As the sun rises, the rays enter the prisms more and more obliquely, and the angle of minimum deviation increases; but since the emergent ray makes the same angle with the refracting edge as the incident ray, it follows that the parhelia will remain on the parhelic circle, while receding from the inner halo.
In order to isolate a polarized pencil of rays with a rhomb of Iceland spar, it is necessary to have a crystal of such a thickness that the emergent streams are separated, so that one may be stopped by a screen.
Plane polarized light gives in general two streams of unequal intensity when examined with a rhomb, and for certain positions of the crystal there isonly one emergent stream.
Hence one of the best methods of determining the magnifying power of a telescope to measure the diameter of the emergent pencil of rays, after the FIG.
The resolving power and the width of the emergent beam fix the optical character of the instrument.
That this is not a necessary characteristic of light was discovered by Christian Huygens, who found that, whereas a stream of sunlight in traversing a rhomb of spar in any but one direction always gives rise to two streams of equal brightness, each of these emergent streams is divided by a second rhomb into two portions having a relative intensity dependent upon the position with respect to one another of the principal planes of the faces of entry into the rhombs - the planes through the axes of the crystals perpendicular to the refracting surfaces.